multistream: Dialer accepts multiple /multistream/1.0.0 headers

[lexnv]

The dialer of the multistream protocol can accept multiple times the multistream V1 header:

litep2p/src/multistream_select/dialer_select.rs

Lines 195 to 197 in cc043b6

	Message::Header(v) if v == HeaderLine::from(*this.version) => {
	*this.state = State::AwaitProtocol { io, protocol };
	}

This may lead to peers keeping connections alive due to state mismatch.

My understanding of the libp2p spec is that the multistream header should be send once, as the first message. Then, peers should negotiate the protocol names they want to serve.

The listener code is correctly awaiting the header once:

litep2p/src/multistream_select/listener_select.rs

Line 149 in cc043b6

*this.state = State::RecvHeader { io };

The issue is also present in the rust-libp2p repo, which is where I suspect our code took inspiration from.

Let's return ProtocolError::InvalidMessage if the header is sent multiple times.

cc @paritytech/networking